176 – II.3. BRASSICA CROPS (BRASSICA SPP.)
from local grain elevators, approximately 3 and 5 kilometres long, identified 287 and
29 plants, respectively, growing at the interface of the rail bed gravel and the tall grass of
the right of way. No plants were located on the rail tracks or in the tall grass of the right
of way. Similarly in Australia, a survey, making 400 observations in 5 × 20 m areas along
4 000 kilometres of roads in oilseed rape growing areas, found B. napus plants in only
31%, 20%, 13% and 9% of the observation points in southern New South Wales,
Western Australia, Victoria and South Australia, respectively. Nearly all the plants were
growing within five metres of the roadside, with the vast majority close to or alongside
the road edge, suggesting they originated from seed dropped from passing vehicles
(Agrisearch, 2001).
In the United Kingdom, Crawley and Brown (1995) found that along undisturbed
roadways, the persistence of B. napus is about three to four years and that the density of
such feral populations is correlated with human activities, such as vehicle transport. In a
three-year assessment of feral populations in Scotland, Wilkinson et al. (1995) found that
the turnover of populations was high, with only 19% of the 1993 population persisting
into 1994 and 12% of the 1994 population persisting into 1995. Crawley and Brown
(1995) obtained similar results in southern England. In a study conducted in Germany
from 2001 to 2004, Dietz-Pfeilstetter, Metge and Schönfeld (2006) found persistence
rates for feral populations of 29% between 2001 and 2002, of 12% between 2002 and
2003 and 80% between 2002 and 2004. However, molecular profiling using ISSR-PCR
(inter-simple sequence repeats-polymerase chain reaction) revealed that plants appearing
in successive years largely belonged to different genotypes, suggesting new seed input
and an even higher turnover of populations.
Reuter et al. (2008) investigated a 500 km² area in the region of Bremen, Germany
and reported average densities of 1.19/km² and 1.68/km^2 of feral and volunteer oilseed
rape populations in rural and urban areas, respectively. The investigation showed that
population density varies between years and feral plants tend to be smaller in stature
(by at least 40%) than plants growing on cultivated land.
Surveys by Agrisearch (2001) and MacDonald and Kuntz (2000) suggest that to
survive spring, B. napus roadside populations need to be regularly replenished. However,
in France, Pessel et al. (2001) found roadside feral populations contained plants of old
varieties that had not been grown for eight to nine years, indicating that the seed source
was not entirely from recent vehicle spillage. These results are in keeping with previous
reports that seed of old rapeseed varieties can persist for at least five to ten years after
they were last reported grown (Squire et al., 1999; Orson, 2002). Pessel et al. (2001)
suggested that the analysed roadside feral populations arose from multiple spillages from
different fields or germination of seed from a mixed seed bank or most likely, both.
In Austria, Pascher et al. (2006) genetically analysed plants from 9 selected feral
populations consisting of 50-150 individuals. They found the feral populations were
genetically more diverse than could be explained by the dominant varieties grown in the
area in the previous five years. They concluded that even though the feral populations
largely reflected the genetic makeup of the dominate varieties being grown, a significant
portion of plants had originated from seed banks older than five years. They also found
that feral populations disappeared more quickly under dense grass cover than at sites with
little vegetation, but genetic diversity remained unchanged. Their results indicated that
genetic migration from commercial varieties to feral populations was five times greater
than the inverse.
